[PATCH] D67244: LangRef: mention MSan's problem with speculative conditional branches.

[Evgenii Stepanov via Phabricator via llvm-commits]

eugenis created this revision.
eugenis added reviewers: vitalybuka, efriedma.
Herald added a project: LLVM.

This short blurb aims to disallow optimizations like we had to revert
(under MSan) in

  https://reviews.llvm.org/D21165
  https://bugs.llvm.org/show_bug.cgi?id=28054
  https://reviews.llvm.org/D67205


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D67244

Files:
  llvm/docs/LangRef.rst


Index: llvm/docs/LangRef.rst
===================================================================
--- llvm/docs/LangRef.rst
+++ llvm/docs/LangRef.rst
@@ -3245,6 +3245,17 @@
 location could clobber arbitrary memory, therefore, it has undefined
 behavior.
 
+**MemorySanitizer**, a detector of uses of uninitialized memory,
+defines a branch with condition that depends on an undef value (or
+certain other values, like ex. a result of a load from heap-allocated
+memory that has never been stored to) to have an externally visible
+side effect. For this reason functions with *sanitize_memory*
+attribute are not allowed to produce such branches "out of thin
+air". More strictly, an optimization that inserts a conditional branch
+is only valid if in all executions where the branch condition has at
+least one undefined bit, the same branch condition is evaluated in the
+input IR as well.
+
 .. _poisonvalues:
 
 Poison Values


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